This invention relates to the joining of materials, and, more particularly, to the wire bonding of articles using applied force and ultrasonic energy.
Wire bonding is a technique for bonding small pieces of metal together. It is widely used in the manufacture of electronic circuits and various devices that require the joining of small metallic pieces under conditions that do not permit extensive heating of the pieces and the surrounding structure, such as would be experienced if joining were accomplished by soldering or brazing. For example, external connections to a semiconductor electronic microcircuit on a silicon chip may be made using fine wires bonded at one end to pads on the microcircuit and at the other end to traces or leads. The microcircuit cannot be substantially heated by the bonding operation, because the electronic structure of the microcircuit could be damaged by the elevated temperature.
In wire bonding, the two pieces to be joined are placed into contact. They may be supported from one side on a substrate, and the substrate rests on a tool, typically a flat plate. A second tool is placed into contact with the other side of the pieces, so that the two pieces to be joined are sandwiched between the two pieces of tooling. The second tool usually has a tip in the form of an inverted truncated cone with a small bonding tip only slightly larger than the desired area of the bonded region. A compressive bonding force is applied between the pieces of tooling, and at the same time the second tool is excited ultrasonically, typically at a frequency of 60-100 KHz (kilohertz). The ultrasonic energy passes through the pieces being joined.
The truncated conical shape of the second tool concentrates the ultrasonic energy in a small area, resulting in a high energy density in that bonding area. The combination of the applied compressive bonding force and the ultrasonic energy cause the two pieces to weld together in the bonding area, completing the required joint. This process, termed "wire bonding" even though the pieces are not necessarily wires, can be completed in 0.1-0.2 seconds or less, transfers little heat into other regions of the pieces being bonded, and is amenable to bonding large numbers of pieces at the same time in different regions. Thus, using commercial automated wire bonding equipment, all of the wire bonds to a typical microcircuit can be completed in a few seconds (depending on the number of bonds required) without damaging the active circuit elements.
Wire bonding is widely used in many situations, but it has drawbacks in other situations. One such problem area is that not all materials respond to the wire bonding operation in the same way. For example, if one or both of the pieces being joined is a "soft" material that tends to deform extensively even without the application of ultrasonic energy, it is found that the wire bond may not be strong. In another example, if the substrate upon which the metallic pieces are supported is soft, it may be difficult to accomplish the wire bonding. As an example, it is difficult to use conventional wire bonding procedures to reproducibly bond metal wires to materials supported on compliant substrates such as Kapton, Nylon, Delron, and Duroid.
Thus, there is a need for an improved procedure for wire bonding of materials, particularly where one or both pieces, or the supporting substrate, is soft and readily deformed. The present invention fulfills this need, and further provides related advantages.